1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to an optical unit for use in a laser beam printer apparatus, and to a method of fixing optical elements in the optical unit.
2. Description of the Related Art
In general, a raster scanning type optical unit for use in a laser beam printer apparatus includes an optical deflecting system for deflecting a light beam, emitted from a light source, toward an image bearing member such as a photoconductor, and a focal system having optical elements for converging the light beam on the photoconductor. The optical deflecting system and focal system are generally housed in a housing.
The optical deflecting system includes, for example, a before-deflection optical system having lenses for transforming the light beam to have a predetermined spot size.
The focal system includes an after-deflection optical system having optical elements such as an f.theta. lens for correcting the distortion and field curve of the light beam deflected by the optical deflector, thereby forming a light beam having a predetermined level.
In the optical unit, the light beam emitted from the light source is transformed by the before-deflection optical system to have a predetermined spot size, is then deflected by the optical deflector toward the after-deflection optical system at a constant angular speed, and is converged by the after-deflection optical system onto the photoconductor.
In general, the f.theta. lens of the after-deflection optical system and any of the optical elements, are secured to the housing by means of different types of mechanical-holding members or by various bonding agents.
The housing for the optical unit is accurately molded out of a thermohardening type resin or a thermoplastic type resin with high accuracy In the housing, nuts or screw members having screw holes formed therein are inserted before molding or press fitted after the molding, for screwing the leaf-springs to secure the optical elements.
In this case, a lot of working steps are required for securing the optical elements to the housing, inevitably making the working process complex. Further, a lot of components including the leaf-springs are required for securing the f.theta. lens and the like to the housing, which increases the manufacturing cost.
To avoid this, a method has been proposed in which the leaf-springs are secured by self-tapping screws in place of the above-described nuts or screw members having screw holes formed therein However, this method is disadvantageous, since cuttings will be caused from the housing itself when the screws are fastened, and the cuttings may interrupt passing of the light beam. This may deteriorate the quality of an image formed on the photoconductor. Thus, the proposed method is not suitable for assembling the optical unit.
On the other hand, in the case of securing the optical elements by means of an epoxy or cyano-acrylate bonding agent, not only a lot of time is required until the agent is hardened, but also a sufficient bonding strength cannot be obtained.
Moreover, in the case of using a thermohardening type bonding agent, heat applied for hardening the agent may deform the housing In this case, if the f.theta. lens is made of PMMA (polymethyl methacrylate) resin, the heat may deform the lenses, as well as the housing. This method is disadvantageous, particularly since it is not suitable to apply the method to a lens made of resin and hence capable of having a complex shape.
In addition, a method using an ultraviolet (UV) hardening type bonding agent for securing the f.theta. lens to the housing has been proposed However, this method cannot provide a sufficient bonding strength, either. Further, it is known that ozone or heat may be generated at the time of hardening of the agent. Lenses made of PMMA or the like are not permeable to ultraviolet rays, so that a longer period of time is required for hardening the agent, and a sufficient bonding strength cannot be obtained What is worse still, positions where bonding agents can be dropped will be limited to those areas where sufficient amount of UV rays are incident.